The 36-year-old signed a one-month deal on Monday and went on to make his debut later that evening in the Seagulls' 1-0 defeat by West Ham United.
Harper told BBC Sussex: "I didn't want to sit and do nothing for the last couple of years of my career.
"I've heard it could be up until January, the longer the better. It's better than doing nothing."
Harper also revealed that other clubs had been interested in taking him on loan, and added: "There'd been a few clubs sniffing. Gus [Poyet, Brighton manager] was first-class on the phone.
"He was watching a match while talking to me, but he still managed to sell the club to me.
"I didn't hesitate, there's a fantastic set up here. The club's just come up, is going places and I thought why not? It's an exciting opportunity."
Harper is Newcastle's longest-serving player but he spent the majority of his time as back-up to Shay Given and has made 190 appearances in his 18 years with the club.
"If I could speak to a young Steve Harper 10 years ago, I would have told him to go on loan then," he continued.
"Everybody knows I haven't played enough football until the last two-and-a-half years.
"I hadn't played a competitive game for about six months so it was nice to blow the cobwebs out."